Holographic memory systems normally employ a page-oriented storage approach. An input device such as a SLM (spatial light modulator) presents recording data in the form of a two dimensional array (referred to as a page), while a detector array such as a CCD camera is used to retrieve the recorded data page upon readout. Other architectures have also been proposed wherein a bit-by-bit recording is employed in lieu of the page-oriented approach. All of these systems, however, suffer from a common drawback in that they require the recording of a huge number of separate holograms in order to fill the memory to capacity. A typical page-oriented system using a megabit-sized array would require the recording of hundreds of thousands of hologram pages to reach the capacity of 100 GB or more. Even with the hologram exposure times of millisecond-order, the total recording time required for filling a 100 GB-order memory may easily amount to at least several tens of minutes, if not hours. Thus, another holographic ROM system such as shown in FIG. 5 has been developed, where the time required to produce a 100 GB-order capacity disc may be reduced to under a minute, and potentially to the order of seconds.
The holographic ROM system in FIG. 5 includes a light source 1, HWPs (half wave plates) 2, 12, an expanding unit 4, a PBS (polarizer beam splitter) 6, polarizers 8, 14, mirrors 10, 16, a mask 22, a holographic medium 20, and a conical mirror 18.
The light source 1 emits a laser beam with a constant wavelength, e.g., a wavelength of 532 nm. The laser beam, which is of only one type of linear polarization, e.g., P-polarization or S-polarization, is provided to the HWP 2. The HWP 2 rotates the polarization of the laser beam by θ degree (preferably 45°). And then, the polarization-rotated laser beam is fed to the expanding unit 4 for expanding the beam size of the laser beam up to a predetermined size. Thereafter, the expanded laser beam is provided to the PBS 6.
The PBS 6, which is manufactured by repeatedly depositing at least two kinds of materials each having a different refractive index, serves to transmit one type of polarized laser beam, e.g., P-polarized beam, and reflect the other type of polarized laser beam, e.g., S-polarized beam. Thus the PBS 6 divides the expanded laser beam into a transmitted laser beam (hereinafter, a signal beam) and a reflected laser beam (hereinafter, a reference beam) having different polarizations, respectively.
The signal beam, e.g., of a P-polarization, is fed to the polarizer 8, which removes imperfectly polarized components of the signal beam and allows only the purely P-polarized component thereof to be transmitted therethrough. And then the signal beam with perfect or purified polarization is reflected by the mirror 10. Thereafter, the reflected signal beam is projected onto the holographic medium 20 via the mask 22. The mask 22, presenting data patterns for recording, functions as an input device, e.g., a spatial light modulator (SLM).
On the other hand, the reference beam is fed to the HWP 12. The HWP 12 converts the polarization of the reference beam such that the polarization of the reference beam becomes identical to that of the signal beam. And then the reference beam with converted polarization is provided to the polarizer 14, wherein the polarization of the reference beam is more purified. And the reference beam with perfect polarization is reflected by the mirror 16. Thereafter, the reflected reference beam is projected onto the conical mirror 18 (the conical mirror 18 being of a circular cone having a circular base with a preset base angle between the circular base and the cone), which is fixed by a holder (not shown). The reflected reference beam is reflected toward the holographic medium 20 by the conical mirror 18. The incident angle of the reflected reference beam on the holographic medium 20 is determined by the base angle of the conical mirror 18.
The holder for fixing the conical mirror 18 should be installed on the bottom side of the conical mirror 18, in order to prevent the reference beam from being blocked by the holder. Since the holder should be placed on the bottom side of the conical mirror 18, it is usually installed through a center opening 24 of the holographic medium 20.
The holographic medium 20 is a disk-shaped material for recording the data patterns. The mask 22 provides the data patterns to be stored in the holographic medium 20. By illuminating the mask 22 with a normally incident plane wave, i.e., the signal beam, and by using the reference beam incident from the opposite side to record holograms in the reflection geometry, the diffracted pattern is recorded in the holographic medium 20. A conical beam shape is chosen to approximate the plane wave reference beam with a constant radial angle at all positions on the disc, such that the hologram can be read locally by a fixed-angle narrow plane wave while the disc is rotating during playback. Furthermore, an angular multiplexing can be realized by using the conical mirror 18 with a different base angle (see “Holographic ROM system for high-speed replication”, 2002 IEEE, by Ernest Chuang, et al.).
By using the above-mentioned scheme, the time required to produce a fully recorded 100 GB-order capacity disc may be reduced to less than a minute, and potentially to an order of seconds.
Meanwhile, in order to record holographic data in the holographic medium 20 (hereinafter, also referred to as “disk”), it is required to precisely align the mask with the disk. In a conventional method of aligning the disk and the mask, an operator directly observes alignment marks formed thereon by using a high multiple microscope and an illuminating device.
However, in such a conventional method, the productivity of the holographic ROM system is decreased since the operator should align the disk and the mask while directly observing the alignment marks thereof with his/her eyes through the microscope. Further, if the operator is not a skilled person, there is high likelihood of a misalignment of the disk and the mask.